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    <title>Document</title>
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    <h4>打印a以内所有偶数的和:</h4>
    <script>
    function add(a){
       var sum = 0;
      for (var i = 1; i <= 100; i++) {
        if (i % 2 == 0) {
          sum += i;
          //   document.write(i + " ");
        }
      }
      document.write(sum);
    }
      var a=prompt("输入一个数据 计算他以内所有偶数的和");
      add(a);
    
    </script>
  </body>
</html>
